Cathode active materials, which are one material constituting lithium secondary batteries, play a critical role in determining battery capacity and performance.
As cathode active materials, lithium cobalt oxides (e.g., LiCoO2) that have relatively excellent overall physical properties such as excellent cycle characteristics and the like are mainly used. However, cobalt used in LiCoO2 is a so-called rare metal and supply of cobalt is unstable because reserves and production thereof are limited. In addition, LiCoO2 is expensive due to unstable supply of cobalt and increasing demand for lithium secondary batteries.
Under these circumstances, research on cathode active materials that can replace LiCoO2 is continuously underway and use of lithium-containing manganese oxides such as LiMnO2, LiMn2O4 having a spinal crystal structure, and the like and lithium-containing nickel oxides (e.g., LiNiO2) is also under consideration. However, it is difficult to apply LiNiO2 to actual mass-production at reasonable costs in terms of characteristics according to a preparation method thereof, and lithium manganese oxides such as LiMnO2, LiMn2O4, and the like have poor cycle characteristics and the like.
Thus, recently, research on a method of using, as a cathode active material, a lithium composite transition metal oxide including at least two transition metals selected from among nickel (Ni), manganese (Mn), and cobalt (Co) or a lithium transition metal phosphate, which are representative alternative materials, has been underway.
In particular, lithium transition metal phosphates are largely divided into LixM2(PO4)3 having a NASICON structure and LiMPO4 having an olivine structure, and have been studied as a material having higher stability at high temperature than existing LiCoO2. Currently, Li3V2(PO4)3 having a NASICON structure is known and, among compounds having an olivine structure, LiFePO4 and Li(Mn, Fe)PO4 are most widely studied.
Among the compounds having an olivine structure, in particular, LiFePO4 has a voltage of ˜3.5 V (vs. lithium), a high bulk density of 3.6 g/cm3, and a theoretical capacity of 170 mAh/g. In addition, LiFePO4 has higher stability at high temperature than Co and uses Fe as a raw material and thus is highly applicable as a cathode active material for lithium secondary batteries in the near future.
Conventional methods of preparing such cathode active materials are largely divided into dry calcination and wet precipitation. According to dry calcination, a cathode active material is prepared by mixing an oxide or hydroxide of a transition metal such as Co or the like with lithium carbonate or lithium hydroxide as a lithium source in a dried state and then calcining the resulting mixture at a high temperature of 700° C. to 1000° C. for 5 to 48 hours. Dry calcination is, advantageously, a widely used technology for preparing metal oxides and thus is easy to approach, but is disadvantageous in that it is difficult to obtain single-phase products due to difficulties in uniform mixing of raw materials and, in the case of multi-component cathode active materials consisting of two or more transition metals, it is difficult to uniformly arrange at least two elements at the atomic level.
In wet precipitation, which is another conventional cathode active material preparation method, a cathode active material is prepared by dissolving a salt containing a transition metal such as Co or the like in water, adding alkali to the solution to precipitate the transition metal in the form of transition metal hydroxide, filtering and drying the precipitate, mixing the resulting precipitate with lithium carbonate or lithium hydroxide as a lithium source in a dried state, and calcining the mixture at a high temperature of 700° C. to 1000° C. for 1 to 48 hours. Wet precipitation is known to easily obtain a uniform mixture by co-precipitating, in particular, two or more transition metal elements, but requires a long period of time in precipitation reaction, is complicated, and incurs generation of waste acids as by-products.
In addition, various methods, such as a sol-gel method, a hydrothermal method, spray pyrolysis, an ion exchange method, and the like, have been used to prepare a cathode active material for lithium secondary batteries.
Meanwhile, a method of preparing cathode active material particles using supercritical water has recently received much attention. JP 2001-163700 discloses a method of preparing a metal oxide for cathode active materials by allowing lithium ions to react with transition metal ions in a supercritical or subcritical state in a batch-type reactor and a continuous reactor. KR 2007-008290, which was filed by the present applicant prior to the filing of the present application, discloses a method of preparing a lithium iron phosphate having an olivine crystal structure using a supercritical hydrothermal method.
However, in existing supercritical devices, a reaction fluid, which is an intermediate product generated due to reaction between raw materials, rapidly gels and thus the reactants are not uniformly mixed. In addition, fluidity of the reaction fluid is deteriorated and thus clogging of the inside of a mixer frequently occurs. As a result of previous studies, it was found that, when a reaction fluid in a gel state is strongly mixed, a sol-state reaction fluid having a uniform mixing state and very high fluidity may be obtained. However, a fixed mixer of a generally used supercritical device is inserted into a tube and thus mixing effects that are strong enough to solate a reaction fluid may not be obtained, and the fixed mixer rather acts as resistance and thus disturbs flow of the reaction fluid and therefore the above-described problems cannot be addressed.
Therefore, there is a high need to develop a technology that addresses the clogging problem by enhancing fluidity of a reaction fluid and enables uniform mixing of raw materials, in preparation of a lithium composite transition metal oxide using supercritical or subcritical water.